Atomic layer deposition (“ALD”), formerly known as atomic layer epitaxy (“ALE”), is a thin film deposition process that has several benefits over other thin film deposition methods, such as physical vapor deposition (“PVD”) (e.g., evaporation or sputtering) and chemical vapor deposition (“CVD”), as described in Atomic Layer Epitaxy (T. Suntola and M. Simpson, eds., Blackie and Son Ltd., Glasgow, 1990).
In contrast to CVD, in which the substrate is exposed to multiple precursors simultaneously present in the reaction chamber, the precursor exposures in ALD processing are sequential, so that the substrate is exposed to only one precursor at a time. Successful ALD growth has conventionally involved the sequential introduction of two or more different precursor vapors into a reaction space around a stationary substrate in a deposition system known as a traveling wave reactor. ALD is conventionally performed at elevated temperatures and low pressures. For example, the reaction space may be heated to between 200° C. and 600° C. and operated at a pressure of between 0.1 mbar and 50 mbar. In a typical traveling wave type ALD reactor, the reaction space is bounded by a reaction chamber sized to accommodate one or more substrates. One or more precursor material delivery systems (also known as “precursor sources”) are typically provided for feeding precursor materials into the reaction chamber.
After the substrates are loaded into the reaction chamber and heated to a desired processing temperature, a first precursor vapor is directed over the substrates. Some of the precursor vapor chemisorbs or adsorbs on the surface of the substrates to make a monolayer. The molecules of precursor vapor will typically not attach to other like molecules and the process is therefore self-limiting. However, some precursors may tend to physisorb or otherwise attach to like molecules at the surface of the substrate in excess non-chemisorbed amounts. After exposure to the first precursor vapor, the reaction space is purged to remove excess amounts of the first vapor and any volatile reaction products. Purging is typically accomplished by flushing the reaction space with an inert purge gas that is non-reactive with the first precursor. For ALD deposition, the purge conditions and duration are sufficient to remove substantially all non-chemisorbed precursor. After purging, a second precursor vapor is introduced. Molecules of the second precursor vapor chemisorb or otherwise react with the chemisorbed first precursor molecules to form a thin film product of the first and second precursors. To complete the ALD cycle, the reaction space is again purged with an inert purge gas to remove any excess of the second vapor as well as any volatile reaction products. The steps of first precursor pulse, purge, second precursor pulse, and purge are typically repeated hundreds or thousands of times until the desired thickness of the film is achieved.
U.S. patent application Ser. No. 11/691,421, filed Mar. 26, 2007 and published as Pub. No. US 2007/0224348 A1 of Dickey et al. (“the '421 application”) describes various methods and systems for atomic layer deposition on flexible substrates. The specification of the '421 application is incorporated herein by reference in its entirety. The '421 application describes ALD deposition methods involving alternating exposure of a substrate to first and second precursor gases without the use of dynamically pulsing precursor and purge gas flows as in conventional traveling wave ALD reactors. In the systems and methods of the '421 application, a substrate such as a flexible web is reciprocatingly moved along an undulating path through two or more precursor chambers or zones separated by one or more isolation chambers or zones to accomplish atomic layer deposition of thin films on the surface of the substrate. As the substrate traverses between the precursor zones, it passes through a series of flow-restricting passageways of an isolation zone into which an inert gas is injected to inhibit migration of precursor gases out of the precursor zones.
The present inventors have discovered that, when the system of the '421 application is used with trimethylaluminum (TMA) and water (H2O) as the first and second precursors to deposit alumina (Al2O3) thin films, for a given set of conditions of water dose strength, source temperature, and zone separation there is a substrate translation speed above which excess non-chemisorbed water molecules seem to be transported with the substrate into the TMA precursor zone. Experimental observations suggesting excess water is being transported into the TMA precursor zone at high speed include an observed increase in the thin film deposition rate per cyclical exposure to the precursors with an attendant decrease in barrier layer properties of the thin film, and deposition of Al2O3 on walls of the TMA precursor zone of the deposition system at the location where the substrate first encounters the TMA precursor vapor. These observations are consistent with CVD-type deposition in the TMA precursor zone.
It is expected that precursors other than water may exhibit similar unwanted transport behavior at some combination of process conditions and high substrate transport speed. For example, a precursor that tends to adhere strongly to itself (e.g., a precursor that has a relatively high surface tension against the inert gas used to isolate the precursors from one another), has a low vapor pressure, and/or that exhibits strong physisorption to the surface of the substrate may be more likely to be transported with the substrate into the other precursor zone.
The present inventors have identified a need to inhibit excess non-chemisorbed amounts of a first precursor from being transported with the substrate into contact with a reactive second precursor, and have devised methods and systems for quickly removing, desorbing, or liberating non-chemisorbed amounts of the first precursor from the surface of the substrate and inhibiting them from migrating into contact with the second precursor.